Process for removing scale from zirconium metal and alloys thereof



Patented Sept. 22, 1953 EXANHNLK PROCESS FOR REMOVING SCALE FROM ZIR-CONIUM METAL AND ALLOYS THEREOF Elmer Donald Billing and Gordon LowellFrederic, Albany, reg., assignors to the United States of America asrepresented by the Secretary oi the Interior No Drawing. ApplicationOctober 10, 1950,

Serial No. 189,472 I 1 Claim.

(Granted under Title 35, U. S. Code (1952),

sec. 266) The invention described herein may be manufactured and used byor for the Government of the United States for governmental purposeswithout the payment to us of of any royalty thereon in accordance withthe provisions of the act of April 30, 1928 (ch. 460, 45 stat. L. 467).

This invention relates to pickling solutions for cleaning and removingscale from zirconium metal and alloys thereof, and more particularly,from zirconium shaped objects which have been hot worked whereby theyhave become coated with a tenacious layer of oxide.

In the hot working of zirconium objects the metallic zirconium,including alloys thereof, reacts with atmospheric gases and becomescoated with a tenaciously adherent scale which must be removed beforefinal finishing. As is customary in the metal pickling art, the objectsare degreased and then pickled in an acidic solution designed todissolve the scale but not substantially to affect the metallic object.In the case of zirconium and its alloys 'it has been exceptionallydiihcult to remove the scale without at the same time pocking orotherwise adversely affecting the sheet, bar or other shaped object ofzirconium.

Accordingly, this invention has for an object the pickling of metallicobjects of zirconium and its alloys without substantially affecting themetallic shaped object. Another object of this invention is to provide apickling bath containing a certain preferred proportion of reagentswhich accomplishes the maximum cleaning or pickling with minimumdestruction of the metallic zirconium itself. Obviously, to beeffective, the pickling bath must selectively dissolve the undesiredscale components while having substantially no adverse action on themetallic suband nitrate in contact with metallic lead.

Peculiarly enough, the presence of metallic lead in the pickling bath ishighly desirable, inasmuch as the highly successful pickling operationof this invention operates to advantage in its presence. Apparentlythere exists a cooperative relationship between the etching or picklingcomponents, the metallic lead and the zirconium metal being pickled,such that the scale loosening and removal is more complete in thepresence 2 of lead, while the undesired attack upon the metalliczirconium itself is substantially inhibited. Thus solution, etching andpocking of the metallic zirconium object being pickled is substantiallyprevented.

In carrying out this invention, there may be employed any of the usualforms of zirconium requiring a pickling treatment, such as hot rolledbars, sheets, rods, or the like, formed of pure zirconium or its usualalloys containing a substantial proportion of zirconium, such as forexample, the alloy containing from 95 to 99 percent zirconium, thebalance being substantially hafnium. If desired, the object of zirconiumto be descaled or otherwise cleaned by pickling can be subjected to apreliminary sand blasting operation as is known to those skilled in theart, but oneof the significant advantages of this invention lies in thefact that preliminary sand blasting may be dispensed with entirely insuitable cases.

A preferred pickling bath is made up containing about wrcentby vplun eof 60 percent hydrofluoric ac1d,about'3 percent by weight of leadnitrate, about 3 erc nt by vglume of 70.percent nitric acid, a su santial excess of free metallic lead over that required by solution inthe acid reagents, and the balance being substantially water. The exactproportions of reagents in the pickling bath can be varied considerably,but it is important that a substantial concentration of lead ions,fluoride ions and nitrate ions be present in contact with metallic leadin an aqueous acidic solution having a pH lying between about pH 0.2 andpH 3.5. During the pickling treatment a substantial proportion of themetallic lead present passes into solution which apparently cooperatesto inhibit substantial solution of the zirconium metal itself.

The following example illustrates how the invention may be carried out,but is to be considered illustrative and not limitative:

Emampla-A sheet of zirconium which had been hot rolled in air at 650 C.was employed as a test specimen, inasmuch as it contained a very hardtenacious black oxide on its surface. The test specimen was immersed ina plastic tank containing an aqueous solution of pickling reagents. Thereagent solution was made up to contain water and 3 percent by volume of60 percent hydrofluoric acid, 3 percent by weight of lead nitrate, 3percent by volume of percent nitric acid and strips of metallic lead,totaling about 10 percent by weight of the test specimen to be treated,were placed in the solution.

The zirconium sheet was first cleaned by sand blasting and thendegreased by solvent treatment and soap and water as is known to thoseskilled in the art. Thereupon, the sheet was immersed in the picklingsolution until a black spongy precipitate was formed on the surfacethereof. The sheet was then removed from the bath and rinsed in clearwater to remove the precipitate and pickling solution. During therinsing gentle rubbing with a sponge or soft cloth completely removedthe precipitate and a bright smooth finish of metallic zirconium wasobtained. The object was later buffed to a final high polish.

It will be apparent that this invention has pro vided a completelysatisfactory way to remove tenacious scale and oxide films from hotworked zirconium objects, including the alloys thereof. Peculiarlyenough, the presence of metallic lead in the pickling bath cooperates tofacilitate scale removal and minimize solution of metallic zirconiumitself, while at the same time substantially inhibiting the pocking orpitting familiar to those skilled in this art. This invention alsoprovides desirable proportions of reagents for accomplishing the mostrapid and complete pickling while minimizing undesired side reactions.

Since many apparently differing embodiments of this invention will occurto those skilled in the art, the foregoing description and exampleshould be considered as illustrative only and not as limiting theinvention to the precise details disclosed and described.

What is claimed is:

A process for removing scale from metallic articles of zirconium and itsalloys that consists in subjecting the article to the action of apickling bath consisting of about 32; by volume of hydrofluoric acid, 3%by volume of 79% nitric acid, 93-94% by vanes of water with about'3'%'by weight of lead nitrate, while maintaining free metalic lead. in thebath, and maintaining such articles in contact with said bath until thescale is loosened and removable by washing, and thereafter washing andrinsing the articles in water.

E. DON DILLING. GORDON LOWELL FREDERIC.

References Cited in the file of this patent UNITED STATES PATENTS Number

